The initial stability of orthodontic mini-implants is well investigated over a period of 6 weeks. There is no clinical data available dealing with the long-term stability. The aim of this study was the assessment of long-term stability of paramedian palatal mini-implants in humans.
Stability of 20 implants was measured after removal of the orthodontic appliance (sliding mechanics for sagittal molar movement 200 cN each side) before explantation (T4) using resonance frequency analysis (RFA). Data were compared with a matched group of 21 mini-implants assessing the stability immediately after insertion, and after 2, 4, and 6 weeks (T0-T3). The mini-implants used in this study were machined self-drilling titanium implants (2.0 × 9.0 mm). Gingival thickness at the insertion site was 1-2 mm.
The implant stability quotient (ISQ) values before removal of the implant at T4 were 25.2 ± 2.9 after 1.7 ± 0.2 years and did not show a statistically significant change over time compared with the initial healing group (T0-T3).
Comparing the stability of mini-implants just after completion of the healing period and at the end of their respective usage period revealed no significant difference. An increase of secondary stability could not be detected. The level of stability seemed to be appropriate for orthodontic anchorage.
Resonance frequency analysis allows long-term monitoring of mini-implant stability.
There was no difference between initial and long-term secondary stability of mini-implants.
In contrast to dental implants, osseointegration will not progress after the healing period.
Skeletal anchorage has proven to be very helpful in critical anchorage situations to either support the reactive unit or obviate the need for a reactive unit altogether. Because of their low invasiveness, mini-implants, introduced by Kanomi, have become an established method to enhance orthodontic anchorage. The failure rate of orthodontic mini-implants is reported to be as high as 16.4%. Well-known factors leading to a high loss rate of mini-implants are insertion at a site with movable mucosa, insertion at a site with insufficient bone quality and quantity, and close proximity to dental roots or root contact. These problems may be avoided in the anterior palate, the region with the best bone supply and thin soft tissue, and where there is little risk of interfering with teeth. In comparison with interradiculary inserted mini-implants, paramedian mini-implants in the anterior palate have a failure rate of 2.1%, indicating clinically sufficient long-term stability. Achieving and maintaining implant stability are prerequisites for successful orthodontic treatment with mini-implants. In dental implantology, resonance frequency analysis (RFA) represents the gold standard to noninvasively assess clinical stability of an implant. An implant’s stability can be measured directly after insertion (primary stability), after completion of the healing period (early secondary stability), and also at any later time during the implant’s lifetime. The transfer of this concept to the stability measurement of orthodontic mini-implants has been established and validated in previous studies that examined primary and early-secondary stability of mini-implants in the anterior palate.
It is well known from dental implantology that stability undergoes significant changes from insertion until observation periods up to 20 months, depending on the ongoing healing process at the bone implant surface and time of loading. For palatal mini-implants the longest observational period has been 6 weeks thus far. Therefore, the aim of this study was to examine the long-term stability of palatal min-implants with an established measurement setup.
A comparison with primary and early secondary stability data from a previous prospective clinical trial was carried out to find out whether significant alterations of stability occur during loading of mini-implants with orthodontic forces over a typical orthodontic treatment period.
Material and methods
All patients treated with paramedian mini-implants in the anterior palate were consecutively asked to participate in the study, just before removal of the mini-implants. Treatment mechanics are described in the data acquisition section. Success of orthodontic treatment was not a selection criterion. Participation in the study was strictly voluntarily. Each patient gave informed written consent. The study was performed according to the Declaration of Helsinki guidelines on experimentation involving human subjects and was approved by the ethical committee.
A basic prerequisite for this study was the comparability of the long-term group and the initial healing group, regarding mini-implant diameter, length, insertion site, and insertion protocol. The insertion site in both groups was the anterior palate 3 mm lateral from the midpalatal suture and slightly distal from the third palatine rugae. The same type of mini-implants was used in the study group and in the control group, BENEFIT Mini-Implants (outer diameter: 2.0 mm; length: 9.0 mm; PSM, Precision Medical Specialities, Tuttlingen, Germany).
The insertion protocol was identical in both groups. After local anesthesia of the anterior palate, gingival thickness was measured using a dental probe. Appropriate gingival thickness was defined between 1 and 2 mm. Predrilling was performed under external water cooling (700 rpm) to a depth of 3 mm using a drill with a diameter of 1.3 mm. The implants were inserted perpendicular to the palatal curvature. Insertion was stopped when the implants’ heads touched the soft tissue. The insertion and predrilling were performed using a surgical machine (ElcoMed SA 200C; W&H Dentalwerk, Bürmoos, Austria). The suprastructure was inserted 2-4 weeks after mini-implant insertion. Mini-implant removal was carried out manually using a manual driven contra-angled handpiece without local or topical anesthesia.
Exclusion criteria were systemic diseases affecting the bone metabolism or wound healing. The mini-implants were visually and clinically inspected for clinical signs of periimplant inflammation. Mini-implants that showed clinical signs of periimplant inflammation tested by bleeding on gentle probing (0.25 N) at the time of investigation were excluded.
A clinical pilot study provided adequate data for a sample size calculation. In this study, a significant change in stability by means of implant stability quotient (ISQ) values was observed from week 2 to week 4 for palatally inserted mini-implants of the same size. The respective RFA data were used for power analysis.
The changes in ISQ values (7.89 ± 5.92), a chosen α level of 0.001, and a power of 0.95 resulted in a required sample size for the treated and the control group of n = 19. Calculation was performed using G*Power 3.1.5 software (University of Kiel, Düsseldorf Germany).
A total of 20 consecutive patients were included in the study. The test group was comprised of 10 males and 10 females of white ancestry with a mean age of 13.1 ± 2.00 years ( Table I ).
|Factor||Initial stability group (n = 21)||Long-term group (n = 20)||P value||Test|
|Age, y||13.72 ± 4.56||13.10 ± 2.00||0.601||Mann-Whitney U|
In the long-term group, RFA was performed after removal of the orthodontic appliance before removal of the implant (sliding mechanics for sagittal molar movement 200 cN/gram/0.72 oz each side; Fig 1 ).
Visual inspection included visual detection of infection-related reddening and swelling. Tests for bleeding on probing were performed with a periodontal probe at 4 sites at each implant. Positive bleeding on probing without signs of marginal bone loss around the implant was recorded as perimucositis.
RFA was performed using the Osstell ISQ device (Osstell, Gothenburg, Sweden) as follows: 3 times parallel to the midpalatal suture and 3 times perpendicular to the midpalatal suture ( Fig 2 ). Mean ISQ values were calculated for each direction and overall value (T4).
The sample from the controlled clinical trial was taken as a comparison group. It consisted of 12 males and 9 females of white ancestry with a mean age of 13.72 ± 0.99 years. In this group, the stability of 2.0 × 9.0 mm paramedian mini-implants was observed over a 6-week healing period. RFA measurements were carried at 4 different points in time: T0, immediately after insertion; T1, 2 weeks after insertion; T2, 4 weeks after insertion; and T3, 6 weeks after insertion.
Chi-square and Mann-Whitney U-tests showed that both groups matched regarding sex distribution and age ( Table I ). To match the initial stability group, a corrected age was used for the long-term stability group: the insertion age for the study group was calculated subtracting the date of birth from the insertion date. Additionally, the vertical bone height and facial pattern (ML-NL) were compared in both groups ( Table II ). Vertical palatal bone height was measured at the level of P1/P2, because this landmark on the lateral cephalogram corresponds to the third palatal rugae.
|Measurement||Initial stability group||Long-term group||t test, P value|
|ML-NL (°)||22.1 ± 4.8||24.1 ± 4.2||0.323|
|Vertical bone height (mm)||5.6 ± 1.3||5.1 ± 1.0||0.296|
The data were tested for normal distribution and equality of variances. Shapiro-Wilk and Levene tests showed that parametric statistics were adequate to analyze mean ISQ values. The mean ISQ values between independent samples were tested with independent t test and ANOVA. ISQ values of different measurement directions (along the suture or perpendicular to the suture) were compared using a paired t test. Statistical significances were tested at P < 0.05(*), P < 0.001(**) and P < 0.0001(***) levels. Statistical analysis was carried out using SPSS statistics version 23 (IBM, Chicago, Ill).
The average time span between insertion and removal of the mini-implants was 1.7 ± 0.2 years. Initial stability at T0 was 26.6 ± 5.3 ISQ in the initial stability group. The ISQ value before removal of the implant at T4 (long-term group) was 25.2 ± 2.9 ISQ and did not show statistical significance. The comparison of RFA values between T3 (24.3 ± 5.4 ISQ) and T4 (25.2 ± 2.9 ISQ) showed a slight increase of stability, which was not statistically significant. Comparing all 4 different dates, ANOVA did not reveal a statistically significant difference P = 0.082 ( Table III ). The resulting development of mini-implant stability over time is shown in Figure 3 . The comparison of each measurement direction at each time did not reveal any statistically significant differences ( Table IV ). Three patients were excluded because of signs of perimucositis.